Add psql's \crosstabview

[anandghegde]

Description

Adds psql's \crosstabview [colV [colH [colD [sortcolH]]]] (#1378, dbcli/pgspecial#142). It runs the query and shows the result as a grid: the values of colV go down the side, the values of colH go across the top, and colD fills the cells.

It follows psql's crosstabview.c:

• Columns can be given by number or by name. Unquoted names are case-folded, and "Quoted" names are kept as written.
• Defaults are first column vertical, second horizontal, third data. The data column must be given when the query returns more than three columns.
• Header values keep their order of first appearance. With sortcolH, the horizontal header is sorted by that integer column: non-integers rank 0 and ties are in name order, as in psql.
• NULL headers and NULL values print as null_string. A cell with no data is empty.
• The error messages are psql's: fewer than three columns, the same column for both headers, column number out of range, unknown or ambiguous name, more than one value for a cell, more than 1600 columns. The warning for extra arguments is also psql's.
• select ... \crosstabview works like \G: PGExecute.run() handles it, and it also ends the buffer in multi-line mode. A bare \crosstabview, or select ...; \crosstabview, re-runs the last query sent to the server, like psql. A statement that returns no rows is shown as usual.

I checked it against psql 17's \crosstabview, using the examples from the psql docs and the error cases, on a Docker Postgres. The grid shape, the header order (including sortcolH) and the error text match. Where pgcli prints differently, it does so by design: SELECT n instead of (n rows), True instead of t, <null> for NULL.

Checklist

• I've added this contribution to the changelog.rst.
• I've added my name to the AUTHORS file (or it's already there).
• I installed pre-commit hooks (pip install pre-commit && pre-commit install).
• I verified that my changes work as expected (this may include manually testing them in your local environment, or in other available environments).
• Please squash merge this pull request (uncheck if you'd like us to merge as multiple commits)